1. Field of the Invention
The present invention relates to an amplifier having a function for compensating for distortion by predistortion technique (which will be called distortion-compensated amplifier using predistortion technique below) and, in particular, to a technology for reducing an influence due to the memory effect.
The technical background of the invention will be described below. However, all of those described below may not be conventional technologies.
For example, in a base station apparatus of a mobile communication system adopting Wide-band Code Division Multiple Access (W-CDMA) as a mobile communication technique, radio signals must be transmitted to the physically remote base station apparatus. Therefore, signals such as multi-carriers must be largely amplified by an amplifier such as a power amplifier (PA).
However, since an amplifier is an analog device, the input/output characteristic is a nonlinear function. Especially, after the amplification limit called the saturation point, the amount of power to be input to the amplifier increases, and substantially constant power is output. The nonlinear output causes nonlinear distortion. Though the signal component in an undesired signal band of a signal sent before amplification can be kept at a low level by a band limiting filter, nonlinear distortion occurs in the signal having passed through the amplifier. Thus, the signal component leaks to the undesired signal band (adjacent channel).
For example, since transmission power is high in a base station apparatus as described above, the magnitude of the power leaking to such an adjacent channel is strictly defined. Therefore, a big issue here is how to reduce the power leak to an adjacent channel. Accordingly, the predistortion technique has been proposed as one of distortion compensation techniques and is going mainstream instead of the feed forward technique because an amplification coefficient has been emphasized recently. The predistortion technique can compensate for distortion of signals output from an amplifier by giving in advance inverse characteristics of the AM—AM conversion and AM-PM conversion, which is nonlinearity of the amplifier, to signals input to the amplifier.
FIG. 8 shows a construction example of a functional block of an amplifier using the predistortion technique, and an operation example thereof will be described.
Once a signal is input to a power detecting portion 81, the power or amplitude of the input signal is detected. Then, the detection result is associated as a reference argument of a distortion compensation table 82 including a memory. The distortion compensation table 82 stores a table for performing distortion compensation by the predistortion technique.
An inverse characteristic of nonlinearity of an amplifier (amplifying portion 84 here) to be compensated is reflected to the table in the distortion compensation table 82. Generally, AM—AM conversion (amplitude) and AM-PM conversion (phase) using the power or amplitude of the input signal as an index is used. A predistorter 83 controls the amplitude and phase of the input signal in accordance with a reference result of the distortion compensation table 82. The signal distorted by the predistortion technique in advance is amplified by the amplifying section 84, and a signal without distortion is output. In order to adapt to an environment such as a temperature variation, the control portion 85 updates the distortion compensation table 82 based on the input signal and/or the output signal of the amplifying section 84.
However, in some distortion causing mechanisms in an amplifier, the current state depends on not only an AM—AM characteristic and AM-PM characteristic of instant power, but also a past state such as the memory effect. Since instant power is the only index for referring a distortion compensation table in some conventional methods, the nonlinear distortion caused by the memory effect cannot be compensated, which is a problem.
More specifically, the memory effect acts as hysteresis to an AM—AM characteristic and AM-PM characteristic, the influence generates unbalanced inter modulation distortion (IMD). Thus, the performance of a predistorter conventionally based on instant power of an input signal is limited. Therefore, an accurate nonlinear inverse characteristic cannot be generated by a conventional predistorter.
Here, as an example of a method for overcoming the influence of the memory effect, a distortion compensation table may be provided which has current instant power and instant power before one unit time (such as one sample) as a reference argument.
FIG. 9 shows an example of an image of the distortion compensation table in consideration of the memory effect. The horizontal axis indicates instant power while the vertical axis indicates distortion compensation control values. Changes for every one unit time are shown in FIG. 9. In other words, the distortion compensation table includes a general distortion compensation table and an order of instant power before one unit time.
For example, when a resolution of instant power is 10 bits, 1024 conventional general tables are required (for 10 bits). However, under this construction, while distortion can be compensated, a huge amount of memory is required. Furthermore, creating tables and learning for adapting to an environment are significantly difficult and may be impossible in reality.
In order to avoid these problems, values in a conventional look-up table (LUT) may be corrected by calculating a differentiation coefficient with respect to one time before and with reference to a correction table. However, since proper amounts of suitable correction depend on instant power with the same differentiation coefficient, the correction table always includes an error. Thus, a decreased amount of memory disadvantageously deteriorates the characteristic, and distortion cannot be compensated completely.
Next, the memory effect will be described in detail.
FIG. 10 shows an example model of an amplifying portion in consideration of an influence of the memory effect (refer to Non-patent Document 1, for example). Distortion occurring in the model cannot be compensated by inverse characteristics of an AM—AM characteristic and AM-PM characteristic, which is a principle of predistortion.
Since the entire construction shown in FIG. 10 is equivalent to an amplifying section in this model, processing portions 91 to 96 are not always provided in the amplifying section as they are.
An amplifier 95 functions as an amplifying section having the AM—AM characteristic and AM-PM characteristic only, that is, an amplifying section not considering the memory effect. Distortion can be completely compensated by the predistortion principle.
Here, power supply voltage of a power amplifier (PA) is desirably constant, but power supply voltage varies as described below. Thus, the memory effect occurs.
In general, distortion of the odd order occurs near a carrier as IMD (IMD3 or IMD5).
On the other hand, a frequency spectrum having a nonlinear component of the even order has a baseband and the double or more frequency component of that of an input signal. Current having a distortion component of the even order occurring in the baseband among them is fed to a drain bias circuit, and the current is converted to voltage signals by parasite inductance existing in a bias circuit and/or output matching circuit in a transistor (such as an FET).
For example, when broadband signals are used as in W-CDMA system, a higher band has distortion of even order. Thus, higher voltage has distortion of the even order, and a drain bias fluctuation is caused. Due to the drain bias fluctuation, a carrier signal is modulated again, and new distortion is caused near the carrier.
When an amplifier affected by inter modulation distortion is modeled, a so-called hysteresis characteristic is exhibited in which multiple output voltages and phases occur with respect to one input due to the memory effect of the inductor.
The fact will be described with reference to a model shown in FIG. 10.
A square law detector (square circuit) 92 detects a distortion component of the second order, which occurs in the base band, of a signal input to the amplifier 95 and is assumed as a current source in the model.
An inductance 93 corresponds to a parasite component included in a bias circuit and/or output matching circuit of the amplifier 95. For example, when a broadband signal is handled as in W-CDMA system, the band of the distortion component of the second order in the base band is increased, and the inductance 93 becomes high impedance. Thus, voltage is generated, and voltage of the bias circuit is changed.
In response to a constant phase change (having a fixed value thereof), an input signal is re-modulated by a fixed phase changer 91 with a voltage change in the bias circuit. The re-modulation is performed by a multiplier 94. A signal of a multiplication result by the multiplier 94 is added, as a memory effect component, to a signal output from a conventional amplifier model (amplifier 95) by an adder 96.
Thus, amplitude modulation of an input signal with a signal of voltage of the inductance 93 generates new distortion of the third order. Then, signals from two parts are synthesized by the adder 96, and an amplifier output signal including the influence of the memory effect is generated.
Here, a distortion component of the second order is only considered for describing the mechanism of memory effect occurrence. However, since, in general, a distortion component of the even order such as the fourth order and the sixth order occurs in the baseband, a bias component also occurs. In this case, the portion corresponding to the square law detector 92 may be replaced by the one of the even power such as the fourth power and the sixth power. Then, like the case of the second order, an output from the portion corresponding to the multiplier 94 is added to a signal output from the conventional amplifier model (amplifier 95). The memory effect cannot be compensated by a predistorter only using instant power as a reference argument like a conventional one.
Here, the constructions of the distortion-compensated amplifier using predistortion technique, an amplifying apparatus, a transmitter, a base station apparatus and so on according to the invention are not limited to the above-described constructions, and various constructions may be used instead. The invention can be provided as a method or technique for implementing processing according to the invention, a program of implementing the method or technique and/or a storage medium storing the program. Furthermore, the invention can be further provided as various apparatus and/or systems.
The field of the application of the invention is not limited to the above-described field but the invention can be applied to various fields.
The invention can be applied to various predistortion techniques.
For example, the invention can be applied to predistortion techniques for a baseband (BB), intermediate frequency (IF) band, or radio frequency (RF) band as a frequency band. The invention may be applied to the various kinds of distortion techniques whereby processing may be digitally performed or may be performed in analog form.
While FIGS. 1 and 8 show examples of a distortion-compensated amplifier using predistortion technique, the invention is not limited thereto. A digital-to-analog (D/A) converter, orthogonal modulator (or orthogonal modulating portion), up-converter, filter and so on may be used as required in accordance with a type of a used technique. The existence of a feedback signal to be sent from the amplifying sections 3 and 84 to the control portions 4 and 85 in order to adapt to an environmental change in FIGS. 1 and 8 is not especially limited and the feedback signal may be used or may not be used. Furthermore, in order to use such a feedback signal, a down-converter, an oscillator, a filter, an orthogonal demodulator (or orthogonal demodulating portion), an analog-to-digital (A/D) converter and so on may be used. In addition, in order to detect distortion, an input signal may be used.
For example, a processor in a hardware resource including processors, memories and so on may execute a control program stored in a read-only memory (ROM) to control different kinds of processing to be performed by a distortion-compensated amplifier using predistortion technique, amplifying apparatus, transmitter, base station apparatus and so on according to the invention. Moreover, each function unit for implementing the processing may be an independent hardware circuit.
The invention can be identified as a computer-readable storage medium such as a Floppy (registered trademark) disk and compact disk (CD) ROM storing the control program or as the program (itself). The processing according to the invention can be implemented by inputting the control program from the storage medium to a computer and causing a processor to execute the control program.
2. Description of the Related Art
For example, in a base station apparatus of a mobile communication system, signals to be transmitted may be amplified by an amplifier in an amplifying section of a transmitter. Furthermore, since nonlinear distortion may occur in the amplifier at some levels of input signals, the distortion may be compensated by a predistortion technique.
However, while distortion occurring in an amplifier ignoring the memory effect can be compensated by a conventional predistortion technique, influences due to the memory effect cannot be compensated, which is a problem. While, in order to solve the problem, various studies have been made (refer to Non-patent Document 1, for example), studies are still not enough and further developments have been demanded.
[Non-Patent Document 1]
“Gusuji Hizumi no Eikyo wo Ukeru Zofukuki ni Taisuru Tekiou Predistorter-Gata Hizumi Hosho (Adaptive Predistorter Type Distortion Compensation For Amplifier Influenced by Even Order Distortion)”, Kawaguchi and Akaiwa, Shingakugiho, MW2002-208 (2003-03), p. 63–66.
As described with reference to the conventional example, further developments of a construction for compensating for distortion occurring in an amplifier considering the memory effect have been demanded in addition to a conventional distortion-compensated amplifier using predistortion technique.